[Classification layer accuracy and area report] issue with unclassified pixel inside map

[janzandr]

Reported by @d-pflugmacher.

If a map contains pixel that are not classified (i.e. not assigned to a class), the reported accuracies are wrong.

In this case, the unclassifed pixel have a value of 0, which isn't assigned to a category (calegory values are 1, 2, ...):

[janzandr]

Problem found: the 0 value pixels introduced an additional (false) stratum, which shifted all the correct strata counts by one position to the left, which messed up the calculations.